This invention relates to photochromic compositions including liquids and polymerizates thereof having good infrared absorption and to methods for making them.
High quality optical lenses are widely made by polymerizing allyl diglycol carbonate, i.e., diethylene glycol bis(allyl carbonate), or mixtures thereof with minor proportions of other monomers such as methyl methacrylate and vinyl acetate in molds. Tinted lenses are generally made by dip-dyeing in heated solutions of a veriety of organic dyes. However, most such lenses, whether tinted or colorless, transmit infrared freely.
U.S. Pat. No. 3,692,688 discloses essentially haze-free optical filters of polymethylmethacrylate containing an in situ reaction product of tungsten hexachloride and stannous chloride which is said to filter near infrared radiation efficiently while retaining considerable transmittance of light in the visible region. It is stated that the magnitude of the mole ratio of stannous chloride to tungsten hexachloride is as important a factor as the concentration of the tungsten hexachloride in affecting the filtering efficiency.
U.S. Pat. No. 3,355,294 discloses photochromic compositions containing a thermoplastic polymer, a metal compound such as tungsten hexachloride or tungsten dioxidedichloride, and a metal salt such as ferric chloride which increases the bleaching rate in darkness. A film cast from a solution of polymethylmethacrylate and tungsten hexachloride in dioxane is said to be photochromic, changing from colorless to blue upon exposure to ultraviolet radiation, but has a slower bleaching rate than one containing ferric chloride. It is stated that the disclosed compositions are not photochromic until they are formed into a definite shaped article, such as by casting.
Applicant has found that although lenses made by the polymerization of allyl diglycol carbonate having tungsten hexachloride dissolved therein are photochromic, becoming blue upon exposure to ultraviolet radiation, and absorb infrared when in the colored state, they are hazy and have a permanent undesirable yellow cast upon removal from the molds.
It has been reported by El-Sayed in the Journal of Physical Chemistry, Vol. 68, pp. 433-434, (1964), that tungsten hexacarbonyl is photochromic when dissolved in certain solvents. However, the compound reported displayed a color shift to yellow, which is not a desirable color for sunglasses.